Conventional methods for applying an adhesive in various patterns on a moving workpiece by roll transfer would include the method disclosed in Japanese Kokai No. 3[1991]-90,602. This conventional method uses the device shown in FIG. 1 and is briefly described for illustrative purposes. In FIG. 1, a hot melt adhesive 27 is stored in a tank 30 in which a first roll 31 is partially immersed. As the first roll 31 rotates, the adhesive 27 adheres to the surface of a first roll 31 which has a predetermined pattern formed as hollows on the surface. Next, an excess of the adhesive on the surface of the first roll 31 is scraped off by a doctor blade 35, such that just enough of the adhesive remains to fill up the hollows.
To continue, the adhesive left only in the hollows of said first roll 31 is transferred to the surface of a second roll 32 that has a smooth uneven surface. At this point, a workpiece 7a is moved into contact with said second roll 32 while under pressure exerted by a pressure roll 36, and in this way the adhesive is transferred from the surface of the second roll 32 to the surface of the workpiece 7a in the form of a predetermined pattern.
There is a disadvantage in the above-mentioned conventional technique of the pattern spreading because of the mutual pulling of the adhesive in the hollows of the first roll 31, or when the predetermined pattern is very fine and detailed, of the pattern becoming interconnected with itself and then transferred to the workpiece as a stain. Moreover, whenever the adhesive adheres to areas other than the hollows on the surface of the first roll 31, the adhesive will end up adhering to the surface of the workpiece 7a as a stain. To avoid these problems, it has been necessary to press a doctor blade 35 against the first roll with considerable force in order to scrape off the excess adhesive on the surface of the first roll 31. As a result, the first roll 31 and doctor blade 35 are subject to extreme wear, thus their service life is short.
Furthermore, the possibility of smearing or spreading the pattern increases as the pattern becomes finer and more elaborate. In this situation, it becomes necessary to more precisely scrape the adhesive off the surface of the first roll 31 in order to avoid the spreading or smearing mentioned above, so the force under which the doctor blade 35 is pressed against the first roll 31 is generally increased. As a result, the service life of the first roll 31 and doctor blade 35 is shortened even more. What is more, in situations like this, it becomes difficult to keep the amounts of adhesive required for transfer in the hollows because of the scraping. Accordingly, in some cases the pattern becomes thinned down to the point that fine and/or elaborate details can no longer be transferred.
Furthermore, the second roll, which in most cases is typically made of silicon rubber, is a roll with a flat surface. Because of this, only the pattern portion consisting of the adhesive that adheres to the silicone rubber surface of the second roll, peels off in a thickness of the order of angstroms as the adhesive is transferred to the surface of the workpiece. However, the peeled amount increases and hollows are formed on the second roll as the transfer is repeated. This is because the adhesive gets into these hollows, so that a sufficient amount of adhesive can no longer be provided on the workpiece; thus, there is the possibility of early failure of transfer coating.
It is therefore desirable to provide a method and an apparatus to obtain a more elaborate and finer pattern that remains clear without losing its shape, to prevent stains from adhering to the workpiece, and to extend the life of the first roll, the doctor blade, and the second roll in the above-mentioned method for the roll transfer coating of adhesives.